Moving coil helical resonator



- June 4, 1968 o. R. WOMACK, JR

MOVING COIL HELICAL RESONATOR Filed June 29, 1966 FIG.I

l TURN .042 INCH AXIAL DISPLACEMENT CORE ROTATION IN TURNS INVENTOR.

DAVID R. WOMACK,JR. BY 3. 0am W HIS ATTORNEY.

United States Patent 3,387,236 MOVING COIL HELICAL RESONATOR David R. Womack, Jr., Lynchhurg, Va, assignor to general Electric Company, a corporation of New ork Fiied June 29, 1966, Ser. No. 561,566 4 Claims. (Cl. 333-82) This invention relates to a tunable resonator; more particularly it relates to a helical resonator in which the resonant frequency is varied by moving the helix.

Distributed constant resonators of the type comprising a cavity and a re-entrant coaxial member are well known and widely used circuit elements, and they are resonant at a frequency for which the cavity and the re-entrant member are a quarter wave length long. In one well known variant form of such a resonator, the standard construction is modified by using a helical coil mounted on a dielectric core as the re-entrant coaxial member. By winding the coaxial member in the form of a helix, the physical size of the device is reduced while yet maintaining the electrical length at a quarter wave length.

In resonant cavities of this type, the distributed inductance and capacitance parameters which establish the resonant frequency are determined by the physical dimensions of the cavity and the coaxial element. It is possible, therefore, to design and build a resonator in which the resonant frequency is precisely and accurately controlled. However, it is often desirable to provide some means for adjusting the resonant frequency, both for providing a device in which the frequency can be varied within a predetermined range and also in order to minimize the requirements for dimensional accuracy in fabrication. To this end, it is customary to provide a variable lumped constant capacitor element within the cavity to allow tuning of the cavity. These variable tuning capacitors have taken various physical forms. For example, one approach is to provide two metallic plates with one of these being movable relative to the other through a threaded shaft extending outside of the cavity. Such a lumped tuning capacitor is illustrated in one form thereof in Patent No. 3,247,475 issued Apr. 19, 1966, which shows a helical resonator and an adjustable capacitor, taking the form of two relatively movable metallic plates, one of which is attached to the helix. An alternate method is to provide a dielectric cap or tuning slug which is moved relative to the helix and the cavity, thereby varying the amount of dielectric between the helix and one wall of the cavity to adjust the capacity between the helix and the cavity wall. FIGS. 2 and 3 of Patent No. 2,753,530 issued July 3, 1956 to A. Horvath, illustrate such an arrangement.

All of these prior-art approaches have shortcomings from the standpoint of cost, ease of fabrication, complexity and bulk. Thus, for example, the movable dielectric cap arrangement shown in the Horvath patent requires an additional component which must move in sliding contact with the wall of the cavity and which must be carefully fabricated with a tapered construction in order to provide any reasonable capacity variations. All in all, this results in a bulky, complicated, and costly arrangement. Similarly, utilization of metallic plates and a movable metallic cap requires that the cavity be drilled, tapped and machined to receive the screw members, thus adding machining operations to the fabrication of the resonator and the helix.

A need, therefore, exists for a helical resonator which is of simple construction, small in size, easy to fabricate, and which permits suitable adjustment of the resonant frequency.

It is, therefore, a primary objective of this invention to provide a helical resonator and an associated adjustable capacitor which is simple to manufacture, low in cost, and small in size.

3,387,236 Patented June 4, 1968 A further objective of this invention is to provide a helical resonator which includes a variable capacitor element without adding additional components to the device.

Still another objective of this invention is to provide a helical resonator with a movable helix for adjusting the capacity and, hence, the resonant frequency of the device.

Other objectives and advantages of the invention will become apparent as the description thereof proceeds.

In carrying out the various objectives of the invention, a helical resonator is provided which includes a cavity and a movable helix positioned within the cavity. The helix is mounted on a rotatable threaded core member extending through the cavity. One end of the helix is positioned adjacent to the end wall of the cavity and the other end extends out of the cavity in such a manner as to prevent the helix from rotating, so that rotation of the threaded core causes the helix to be displaced axially along the core. The free end of the helical coil is thus moved with respect to the cavity end wall, thereby varying the capacity between the helix and the end wall, and hence, the resonant frequency. In this manner, an adjustable helical resonator is provided without the use of additional lumped capacitive elements, simplifying the construction of the device and minimizing its cost.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a schematic illustration of a helical resonator constructed in accordance with the invention;

FIGURE 2 is a graph showing the relationship between the resonant frequency of the device and the position of the helix within the resonator.

A tunable helical resonator constructed in accordance with the invention is illustrated schematically in FIG- URE l and includes a metallic outer housing 1 which may be cylindrical or rectangular in shape. The housing defines a cavity in which an adjustable helical coil 2 is mounted on a threaded dielectric core 3 extending along the axis of the cavity and through the housing. The helix 2. may be of copper, or silver-plated copper wire, for example, and has a free end 4 positioned a fixed distance from lower end wall 5 of the cavity. The other helix end 6 extends through an insulating closure memher 7 and is connected to an external input terminal, not shown. A clearance hole 8 in closure member 7 provides passage for the other end of the helix, and, as will be apparent from observation, permits axial movement of portion '6, while constraining rotational movement of the whole coil. An output coupling slot 9 is provided in one wall of the cavity to transfer energy from the resonantv cavity to a suitable output circuit which may me another cavity, etc. It will also be obvious that couplingdevices such as loops, probes or other arrangements may be provided to couple energy from the resonant cavity to a utilization circuit.

Coil end 4'and end wall 5 form a capacitor with the relative distance between the coil and the end wall determining the capacitance between there two elements. By adjusting the position of the coil with respect to the end wall, the resonant frequency of the cavity may be varied without providing a separate capacitor element within the cavity. To this end, the upper end of the threaded rod which extends outside of the housing is provided with a tool receiving slot 10 to permit rotation of the core by use of a tool such as a screw driver. Rotation of the dielectric core produces axial movement of helix 2 within the cavity, since the helix is constrained from rotating with the core. Thus, one complete rotation of core 3 produces an axial displacement equal to the pitch of the coil, thereby varying the capacitance of the capacitor formed by coil end 4 and walls and the resonant frequency of the cavity.

The threaded core member 3 may be constructed of any suitable dielectric material which has a low dielectric constant (as close to the dielectric constant of air as possible) and a low-loss factor for RF energy. One such dielectric material is polystyrene, and one particular form of such polystyrene which has been found to be effective is that marketed under the trade name Rexolite by the Brand-Rex Division of the American Enka Corp.

A helical resonator having the following characteristics was constructed and tested:

Housing, rectangular .74 x .74" x 1.67" Helix:

O.D. incheS .312 Pitch do .042 No. turns 23 The helix was positioned on the core such that at zero or start position, the free end of the helix was .950 inch from the bottom of the cavity, at which position the resonant frequency was 123 megacycles. Each full rotation of the core, therefore, moves the helix .042 inch axially (i.e., a distance equal to the pitch of the coil) in a direction of rotation of the core.

The graph of FIGURE 2 illustrates the variations in the resonant frequency of this resonant cavity as a function of coil position, i.e., the rotation being such that the free end of the coil moved from a position .950 inch away from the end wall to a position .362 inch from the end wall. In the curve of FIGURE 2, the frequency in megacycles is plotted along the ordinate and the coil rotation (in number of turns) in plotted along the abscissa. The coil rotation is plotted from the nominal zero or start position of the coil, i.e., turns represents a coil position .950 inch from the wall, with the coil rotations being such as to bring the free coil end closer to the wall. It may be seen from the curve of FIGURE 2 with the coil in a position with its free end .950 inch away from the end wall, the resonant frequency of the cavity is 123 megacycles. As the core is rotated, thereby axially moving the coil .042 inch for every rotation of the core, the frequency increases until, with 14 full rotations representing an axial displacement of .588 inch of the coil, the resonant frequency is 154 megacycles. It can be seen, therefore, that a frequency range of 31 megacycles can be achieved with a movement of less than a half-inch in at least 14 discrete steps, thus providing an easily adjustable resonant cavity which is simple in construction and easy to manufacture at a reasonable cost.

While a particular embodiment of this invention has been shown, it will, of course, me understood that the invention is not limited thereto, since many modifications both in the circuit arrangement and in the construction may be made. It is contemplated by the appended claims to cover any such modifications as wall within the true spirit and scope of this invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

I. A tunable helical resonator comprising (a) a conductive housing defining a cavity having a wall portion and end portions,

(b) a threaded core of dielectric material extending into the cavity,

(c) a helical conductor disposed on said core and coaxial with said cavity, said helical conductor having a free end spaced from one end of said cavity to form a capacitor element with said end, the other end of said helix extending outside of said housing, to restrain rotation of said helix,

(d) and means to rotate said helix supporting core to cause a lateral translation of said helix along said core for varying the distance of said free end from the end of said cavity to vary the capacity and the resonant frequency of said resonator.

2. The helical resonator according to claim 1 wherein said core extends along the entire length of said cavity and a portion thereof extends outside of said cavity.

3. The helical resonator according to claim 2, wherein the portion of said core extending outside of said cavity is shaped to receive a tool for rotation of said core.

4. The helical resonator according to claim 1 wherein one end of the cavity defined by said metallic housing is enclosed by a non-metallic closure member, a first passage in said closure member to receive said core and a second passage to receive the other end of said helix to constrain rotation of said helix upon rotation of said core but to allow axial movement thereof.

References Cited UNITED STATES PATENTS 3,159,803 12/1964 Czubiak et a1 333-82 HERMAN KARL SAALBACH, Primary Examiner. L. ALLAHUT, Assistant Examiner. 

1. A TUNABLE HELICAL RESONATOR COMPRISING (A) A CONDUCTIVE HOUSING DEFINING A CAVITY HAVING A WALL PORTION AND END PORTIONS, (B) A THREADED CORE OF DIELECTRIC MATERIAL EXTENDING INTO THE CAVITY, (C) A HELICAL CONDUCTOR DISPOSED ON SAID CORE AND COAXIAL WITH SAID CAVITY, SAID HELICAL CONDUCTOR HAVING A FREE END SPACED FROM ONE END OF SAID CAVITY TO FORM A CAPACITOR ELEMENT WITH SAID END, THE OTHER END OF SAID HELIX EXTENDING OUTSIDE OF SAID HOUSING, TO RESTRAIN ROTATION OF SAID HELIX, 